The secondary battery in which an alkali metal such as lithium is used as an active material of a negative electrode has generally various advantages. For example, it not only ensures high energy density and high electromotive force, but also has wide operating temperature range due to the use of a nonaqueous electrolyte. Further, the secondary battery is excellent in shelf life, miniaturized and lightweight.
Therefore, the practical use of the above lithium secondary battery with a nonaqueous electrolyte is anticipated as a power source for use in a portable electronic appliance and also as a high-performance battery for use in an electric vehicle and electricity storage.
However, all the developed prototype batteries have not fully realized the above properties anticipated from the lithium secondary battery, and thus have been incomplete from the viewpoint of charge and discharge capacities, cycle life and energy density.
A major cause thereof resided in a negative electrode used in the secondary battery.
For example, a lithium secondary battery having a negative electrode composed of metal lithium incorporated therein had disadvantageously short cycle life and poor performance stability because lithium precipitated on the surface of the negative electrode during charging formed acicular dendrite causing short-circuit to be likely to occur between the negative and positive electrodes.
Lithium has extremely high reactivity, thereby causing the electrolyte to suffer from decomposition reaction in the vicinity of the surface of the negative electrode. Thus, there was the danger that the above decomposition reaction would modify the surface of the negative electrode to thereby cause repeated uses of the secondary battery to lower the cell capacity.
Various studies have been made on the material of the negative electrode with a view toward obviating the above problems of the lithium secondary battery.
For example, the use of alloys containing lithium, such as lithium/aluminum and Wood's alloy, as the material of the negative electrode of the lithium secondary battery has been studied. However, this negative electrode composed of such a lithium alloy had a problem of crystal structure change attributed to the difference in operating temperature and charge and discharge conditions.
Further, the use of carbon or graphite materials as the material of the negative electrode of the lithium secondary battery has been studied.
For example, an attempt has been made to capture lithium ions formed during charging between graphite layers of a carbon or graphite material (intercalation) to thereby produce a compound known as "intercalation compound" for the purpose of preventing the formation of dendrite.
Carbon fibers derived from coal, coke and PAN and isotropic pitch-based carbon fibers have been extensively studied as the above carbon materials.
However, these carbon materials have several drawbacks, for example, in that not only are graphite crystallites small but also the crystals are disorderly arranged, so that the charge and discharge capacities thereof are unsatisfactory and, when the current density is set high at the time of charging or discharging, decomposition of the electrolyte occurs to thereby lower the cycle life.
Graphite materials such as natural and artificial graphites are now attracting most intensive attention as the material of the negative electrode for use in the lithium secondary battery and are being extensively studied.
Although the chargeable or dischargeable capacity per weight of the natural graphite is pretty large if the graphitization degree is high, the natural graphite has drawbacks in that the current density ensuring ready discharge is low and in that the charging and discharging at a high current density would lower the charge and discharge efficiency. This natural graphite material is not suitable for use in a negative electrode of a high-load power source from which a large amount of current must be discharged and into which it is desired to effect charging at a high current density in order to cut charging time, e.g., a power source for a device equipped with a drive motor or the like.
Also, although the inter-graphite-layer volume as a whole is so satisfactory in the negative electrode composed of the conventional artificial graphite as long as the graphitization degree is high that large charge and discharge capacities are obtained, the artificial graphite has not been suitable for charging and discharging at a high current density.
In the contemporary lithium secondary battery in which use is made of the negative electrode comprising the graphite material, the current density at the time of charging is generally in the range of 20 to 35 mA/g, and thus the charging takes about 10 hours in view of the charge capacity. If the charging can be performed at a higher current density, for example, 100 mA/g, however, the charging time can be as short as 3 hours. Further, if the current density is 600 mA/g, the charging time can be even as short as 30 minutes.
It has been reported that, among the above graphite materials which include natural and artificial graphites, a product of graphitization of carbon fiber derived from mesophase pitch as a starting material (hereinafter referred to as "graphite fiber") is superior in light of the results of measurement of various battery properties, as disclosed in Japanese Patent Laid-Open Publication No. 6(1994)-168725.
However, the carbon materials are various in the size and configuration of crystallites, the content of impurities, etc., depending on the type of the starting material and the manufacturing conditions. Thus, with respect to the above graphite fiber as well, it can hardly be stated that control optimum for the carbon material for lithium-ion secondary battery is being effected.
In recent years, attention is drawn to the relationship of the graphite layered structure or internal texture of a carbon fiber with the cycle life and charge and discharge characteristics of the lithium-ion secondary battery using the negative electrode formed of the carbon fiber, and a multiplicity of reports have been presented. It has now become apparent that the mesophase pitch-based carbon fiber does not necessarily possess the graphite layered structure or internal texture optimum for use in the negative electrode of lithium-ion secondary battery.
For example, J. Electrochem. Soc., 140, 315 (1993) reports that the orientation of internal graphite layers of the graphite fiber used in the negative electrode of the secondary battery has a conspicuous effect on the battery performance. This literature evaluates each of the straight radial texture in which the graphite layers are radially arranged, the flexed radial texture in which minutely flexed (zigzag) graphite layers are radially arranged, the double structure in which graphite layers that minutely flex at the outer-shell part but are linear at the core part are radially arranged and the concentric texture (onion structure) in which graphite layers concentrically lie one upon another, in a plane perpendicular to the axis of the graphite fiber.
As indicated in the literature, the entry of lithium ions from a circumferential surface is difficult in the carbon fiber having the structure in which the texture of graphite layers has concentrically grown in the fiber section, generally known as the onion structure, for example, the vapor-phase grown carbon fiber. Therefore, in the negative electrode formed of this carbon fiber, lithium ions enter and exit only through the fiber cross-section, so that an increase of charge and discharge velocity is accompanied by a conspicuous capacity lowering.
Consequently, the method is contemplated in which the fiber length is reduced to thereby maximize the section surface area per fiber so that the entry and exit of lithium ions through the section are facilitated.
However, reckless pulverization of the fiber for reducing the fiber length causes unfavorable exposure of active graphite layer leading to reaction with the electrolyte, so that disadvantages such as capacity lowering would result.
Further, the method is known in which activation is carried out under special conditions so as to provide the surface of the fiber with pores permitting the entry of lithium ions, as disclosed in Japanese Patent Laid-Open Publication No. 7(1995)-57724. However, in this method, there is the possibility that oxygenic functional groups are formed during the activation, which react with the electrolyte. Therefore, the use of the carbon fiber of the onion structure in the negative electrode has various problems to be solved.
In J. Electrochem. Soc., 140, 315 (1993), it is taught that the graphite fiber of flexed radial texture would lead to exhibition of good battery performance from the viewpoint that lithium ions can enter through the circumferential surface of the fiber and that the fiber is resistant to the destruction attributed to the repetition of expansion and shrinkage of the fiber which is effected by the intercalation and deintercalation of lithium ions.
However, as a result of the inventors' detailed studies, it has become apparent that, with respect to the above carbon fiber of flexed radial texture as well, the initial charge and discharge efficiency (discharge capacity at the first cycle/charge capacity at the first cycle) is low and a capacity lowering (cycle deterioration) is recognized upon the repetition of charge and discharge for a prolonged period of time.
The applicant of this application previously proposed a graphite fiber having a graphite layered structure in which each end of gaps between graphite layers is exposed on the circumferential surface of the graphite fiber and also a material for a negative electrode of lithium-ion secondary battery which includes the above graphite fiber (reference is made to Japanese Patent Application No. 6(1994)-85246). Although the secondary battery including this material for a negative electrode of lithium-ion secondary battery exhibit excellent battery performances such as large charge and discharge capacities and capability of having the current density set high at the time of charge and discharge, the graphite layered structure still has the possibility that the graphite layers deteriorate because of lithium intercalation and deintercalation executed for a relatively prolonged period of time, which thereby deteriorates the battery performances.
Therefore, no carbon fiber has yet been obtained which can resolve all the problems of the conventional material for a negative electrode of lithium-ion secondary battery, such as small charge and discharge capacities, low charge and discharge efficiency at the initial stage, low charge and discharge velocities and short cycle life, and the development of such a carbon fiber is desired.
The inventors have made extensive and intensive studies on the texture and structure of carbon fibers with a view toward resolving the above problems of the prior art. As a result, it has been found effective in improving the charge and discharge efficiency and cycle characteristics of the secondary battery to make a texture of a carbon fiber composed of graphite layers having minute flexures and to laminate these graphite layers to each other so as to orient in a specified direction at an outer-shell part forming a circumferential surface of the fiber. The present invention has been completed on the basis of this finding.